The present invention relates to methods and apparatus for selectively changing a circuit configuration of a power amplifier stage, e.g., for a guitar amplifier, between class A and class A/B.
Designers of guitar amplifiers, up until the late 1950's, focused on creating inexpensive amplifiers with very little distortion. This was not an easy task as solid state devices were not generally available and nearly all amplifiers were designed and built using vacuum tubes. Although tube amplifiers inherently have more distortion than solid state amplifiers, the early amplifier pioneers tried in vain to produce amplifiers with little or no distortion. Most of their designs had a high order of distortion, especially second harmonic distortion, because of the fact that most used a Class A power amplifier topology.
In the context of electric guitar amplification, the existence of harmonic distortion in the amplifier is not undesirable. Indeed, it has been generally thought that even order harmonics are pleasing to the ear, (especially the second harmonic), while odd order harmonics are thought to sound “raucous,” “raspy,” and unpleasant, especially the third and fifth harmonics. The desirability of even order harmonics appears to hold even in fairly high ratios to the normal (undistorted) signal.
As most rock guitar players use solid body guitars, which provide relatively little resonance, most of the harmonics associated with the characteristic sounds identifiable as an electric guitar actually come from the amplifier and not the guitar. The harmonics introduced by the guitar amplifier differentiate one guitar system (e.g., guitar and amplifier combination) from another, thereby giving each system its own “personality.” Thus, guitar amplification has become a niche technology within the broader universe of the audio amplification arts. What sets a guitar amplifier apart from an amplifier found in, for example, a sound system or a home stereo is that the latter seeks to reproduce sounds as closely as possible to the input signal, while the former significantly alters the sound and is, in essence, an extension of the instrument and a part of an overall guitar system.
For efficiency, transformer design, and other cost considerations, most high-power tube type guitar amplifiers utilize push-pull output stages operating in class A/B. The A/B class dictates that the output signal is shared by two or more output tubes which are driven by signals that are equal in magnitude and 180 degrees out of phase. In contrast, class A amplifiers are typically single-ended and are driven from a single drive signal. When driven into clipping, a class A amplifier produces an output rich in second order harmonics, which as discussed above is generally considered to be desirable. Unfortunately, a class A/B power amplifier can produce an output rich in odd order harmonics (in addition to some even order harmonics), which is not considered as desirable.
While the design of a class A/B amplifier in accord with conventional wisdom may be acceptable to some users, many guitar players may find the odd-order harmonic content objectionable. It would be desirable, therefore, to provide an amplifier that can be selectively switched between class A and class A/B operation so that either of the following results can be obtained: (i) efficient amplification (at the expense of harmonic content); or (ii) desirable harmonic content (at the expense of efficiency). Finding circuit solutions to achieve selective switching between class A and class A/B operation is not trivial. Indeed, cost, product size and circuit performance may be significantly impacted by the circuit implementation and control methodology employed. It has been discovered that desirable results in cost, size and performance may be achieved employing one or more aspects of the various embodiments of the invention as described and claimed herein.